New York authorities said yesterday that they have identified the two other suspects in last week's murder of a witness in the investigation of Labor Secretary Raymond J. Donovan and have been keeping them under surveillance.

"We are looking at a lot of people, including those two," Bronx District Attorney Mario Merola said yesterday.

Investigators say they believe that one of these two, who are still at large in the mob-style slaying of Nat Masselli, 31, on a Bronx street, is the triggerman who fired a .38-cal. bullet into the back of Masselli's head from the back seat of Masselli's car.

The one suspect arrested so far, Salvatore Odierno, 66, of Valley Stream, N.Y., was identified by witnesses, sources say, as having been sitting in the front seat on the passenger side of Masselli's Lincoln Continental, which Masselli was driving.

Investigators say they believe that Odierno and the suspected triggerman jumped from Masselli's Lincoln and fled in a red Pontiac, parked nearby and driven by the third suspect.

Last night, police said, Odierno was identified in a police lineup as one of the three men believed involved.

"More likely than not, especially judging by the age and description" furnished by witnesses, "the guy in the back seat did it," one investigator said.

FBI agents have been working with New York authorities in an effort to determine whether the shooting was related to the Donovan investigation, which was reopened this summer by special prosecutor Leon Silverman.

Silverman has been investigating fresh allegations about Donovan's conduct as a New Jersey construction company executive, including a renewed charge that Donovan met in Miami in January, 1979, with several gangland figures, among them Nat Masselli's father, William P. Masselli, and a convicted racketeer named Albert Facchiano, both reputed members of the Genovese family of the Mafia.

Nat Masselli was killed Aug. 25, less than 48 hours before his father was scheduled to be called before a federal grand jury assigned to the Donovan inquiry. The appearance was postponed, but William Masselli, who is serving a seven-year federal prison term, is expected to testify today or tomorrow.

Facchiano, 71, serving a prison term in Alabama, was taken to New York Aug. 26 on a writ obtained by Silverman, and evidently appeared before the grand jury Monday.

Both Massellis were questioned in the initial phases of the investigation, and Nat Masselli helped the FBI record four of his telephone conversations with a lawyer from Donovan's company. Silverman said yesterday that he doesn't believe "that's the reason" he was murdered.

District Attorney Merola did not indicate whether any additional arrests in the Nat Masselli killing are imminent, but by Friday he must charge Odierno, who is being held without bond, or release him.